1. Field of the Invention
The present invention is related generally to apparatus and methods for high fidelity sound reproduction, and more particularly to systems and methods for efficiently modifying signal characteristics in different frequency bands in a multi-driver, multi-speaker audio system, especially one installed in a vehicle.
2. Description of Related Technology
Electromechanical transducers such as loudspeakers and other audio drivers are not able to provide accurate, uniform output with respect to frequency response and sound pressure level. Traditional audio drivers are invariably limited to a relatively narrow frequency range, their performance often being compromised in an effort to extend their audio bandwidth. In virtually every case, the greater the bandwidth of the audio driver, the larger the degradation must be to the audio driver's performance.
For example, a 15" diameter audio driver (woofer) has mechanical characteristics such that it has significant difficulty in reproducing a 20,000 Hz signal, although it may offer uniform response at lower frequencies in the range of say 1kHz down to 50Hz. This is primarily due to the audio driver's inherent mass and compliance (mechanical resistance). At the other extreme, a driver of approximately 0.5" diameter (tweeter) cannot accurately reproduce a 50Hz signal because it cannot generate sufficient pressure variations in moving air at such low frequencies. This then explains why there are no single driver, high performance, full range, high fidelity loudspeakers. Generally, the greater the quantity of different individual drivers used in a loudspeaker system the higher the level of potentially attainable performance. Naturally there are physical, financial and practical limits on the total number of actual drivers that can be used in a typical high fidelity system.
High fidelity sound reproduction typically is measured in terms of flatness of response across the audible spectrum, usually 20Hz to 20kHz. Few adults are capable of sensitive perception across the entire range, and there will always be individual preferences as to accentuation of certain frequency characteristics (such as a juvenile desire for excessive bass). Practically always, however, there must be a smooth transition between different frequency bands.
When a high fidelity system is installed in a vehicle, however, special problems are introduced because of the small internal vehicle volume and the limited locations for speaker and electronic circuit installation. Sound waves from any given speaker travel typically only a relatively few feet before encountering a reflecting or partially absorbing surface and being diverted in another direction toward another surface. The direct and internal reflections introduce phase reinforcements and cancellations which give rise to resonances and nulls at virtually arbitrary frequencies throughout the band. These must be equalized in some manner if the potential of the system is to be realized, and so it is now quite common in vehicle sound systems to employ graphic equalizers and electronic crossover circuits. As presently employed, however, these techniques have definite limitations, whether used separately or together. The graphic equalizer, for example, enables amplitude adjustment of frequency slices, but these are predetermined and fixed. The electronic crossovers function to shift the center frequencies and end limits of a frequency band, but this does not provide the flexibility now needed.
There has been for some time a growing trend toward the use not only of separate speakers, but also separate amplifiers receiving signals in different channels. This applies to both newly installed systems and modifications of existing sound systems. When adding more speakers, such as tweeter, woofer or subwoofer, new resonance and crossover problems must be overcome, arising from the nature of the component, its relation to other components and its placement in the vehicle. Prior art systems do- not provide enough flexibility to make the numerous and subtle adjustments that are needed in installing and expanding a system.
It should be understood that just as with high fidelity fanciers for home applications, there has been a constant tendency toward more elaborate and more precise vehicular installations. Not only are separate component systems offered as original equipment with new vehicles; purchasers desire more power, or more speakers, or better performance, or any combination of these for existing installations. The present invention affords the flexibility and adaptability needed to upgrade under a wide variety of conditions.
One of the most common techniques for flattening the frequency response characteristics in a vehicle is to utilize graphic equalizers, centered at frequencies that are spaced one-third octave apart. Thus, three equalizers are used to cover the band of 10kHz to 20kHz, three are used for the 5kHz to 10kHz band, three are used for the 2.5 to 5kHz band and so forth. More than 30 graphic equalizers may have to be used, and because these cover fixed frequency ranges and there is no assurance that a resonance or a null will occur in the center of a range, it can be difficult to achieve suitably precise flatness in frequency response even with this system.